Page 269 - Biology_F5

Biology for Advanced Level Secondary Schools

numbers 700 and 680 stand for peak of Non-cyclic photophosphorylation
wavelength of light which excite electrons Task 5.4
in these pigments. This implies that the Search from internet sources
respective absorption peak for P700 and simulation showing non-cyclic
P680 is at the wavelengths of 700 nm photophosphorylation. Note down
and 680 nm, respectively. Structurally, what you have observed.
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photosystem is a complex of chlorophyll,
accessory pigments, proteins, and other The mechanism of non-cyclic
molecules. They are important in the photophosphorylation involves both
absorption and transfer of light energy photosystems I and II in the movement
through a series of reduction oxidation of electrons to produce ATP. When the
(redox) reactions.
chlorophyll in Photosystem II absorbs light,
releases an excited pair of electrons which
Light reaction uses both photosystems I are transferred to a series of electron acceptor
and II, which are embedded in the thylakoid and carrier systems, including cytochromes.
membrane. Essentially, light reaction As the electrons pass through different carrier
starts in photosystem II. Light energy systems, they release energy, but these
(or photons) is absorbed by photosystem electrons are not returned to their original
II. The absorbed energy is transferred to position in the photosystem II rather they
the chlorophyll centre, causing electrons are handed over to the photosystem I. The
in the chlorophyll centre to be energised. energy released in a series of electron flow
The energised electrons are excited within system is used by an enzyme called ATP
the chlorophyll molecule; consequently, synthase located in the thylakoid membrane
they move from ground to a higher to synthesise ATP by binding inorganic
energy state. This process is known as phosphate to ADP. This process is called
photo activation. The excited electrons phosphorylation, which literally means
are progressively passed on from one phosphate addition. However, the energy
chlorophyll molecule to the subsequent used in phosphorylation emanates from
the sunlight. Hence, this ATP synthesis is
one via a series of electron carriers to precisely described as photophosphorylation.
a nearby electron acceptor molecule It should be noted that as the two excited
found in the electron transfer chain. The electrons leave the photosystem II in the
movement of electrons can be in one first stage after being activated by the light
direction, whereby they do not move back of wavelength 680 nm, they are replaced
to their original position in photosystem immediately by electrons released from
II but progress to photosystem I, the the splitting of water molecules within the
process which is described as non- cyclic thylakoid. The water molecule splits into
photophosphorylation. On the other hand, hydrogen ion or proton (H ), oxygen and
+
the electrons can move back to their original two electrons (H O →2H + ½O + 2e ).
+
-
2
2
position in photosystem I, in the process This process is called photolysis, because
called cyclic photophosphorylation. it only occurs in the presence of light. Thus,
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